This invention pertains to a three-stage, four-section telescopic hydraulic ram which is especially adapted for use in conjunction with a triple-lift-type mast in a lift truck.
Many industrial lift trucks are equipped with a so-called triple-lift-type, vertically extensible-contractible mast. Such masts include inner, intermediate and outer sections which are extensible vertically relative to one another, with the inner section carrying a carriage on which any one of a variety of load-handling attachments, such as lift forks, may be mounted. The carriage is disposed for travel up and down on and along the inner mast section during what is normally referred to as the "free-lift" operating stage of the truck. Raising and lowering of such a mast is accomplished through mechanism substantially always including a multi-section hydraulic ram, with various sheaves and chains used to provide operative interconnections between the ram and the mast sections.
A number of features are usually sought in such an arrangement. As an illustration, it is important that, during times when a lift truck is being driven and maneuvered from one location to another, the operator have good forward visibility. Under such a driving situation, it is usual that a mast and ram are substantially fully contracted or lowered.
Further, and in order to avoid jolting of a load as it is being raised or lowered, it is desirable that the raising and lowering speed of the carriage, throughout the entire vertical range of travel of the carriage, be as constant as possible. Jolting will occur, of course, if any abrupt change in speed takes place. Obviously, such jolting presents the serious hazard of dropping a load, and causing personal injury and/or other damage.
A general object of the present invention, therefore, is to provide a novel multistage ram which is capable of being incorporated with a triple-lift-type mast in a lift truck in a manner which will take all of the above-mentioned concerns and considerations into account in a practical and satisfactory manner.
According to a preferred embodiment of the invention, the proposed ram comprises a single-acting, four-section, three-stage construction, including a cylinder, an outer large piston slideably received within and extensible from the cylinder, an intermediate piston slideably mounted within and extensible from the outer piston, and an inner piston slideably mounted within and extensible from the intermediate piston.
Provided within the ram of the invention are plural internal fluid passages, with valving mounted in certain of these passages, which direct fluid flow within the ram so as to produce a substantially uniform overall extension speed for the ram when pressure fluid is supplied thereto at a substantially constant rate. The details of construction and location of these passages will be described fully in the description of the ram presented below.
A further important feature of a preferred embodiment of the ram (as disclosed herein) is that it offers an extremely simple construction which is inexpensive to manufacture, and which further presents few maintenance concerns.
The ram of the instant invention can readily be incorporated with a triple-lift-type mast to produce the desired kind of smooth performance mentioned above as being important. The fact that the ram includes four sections ensures its having a relatively short overall length when fully contracted--a condition which contributes significantly to good forward visibility for a truck operator (with the truck and mast in the usual configuration used during transporting of a load).
These and other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.